(a) Field of the Invention:
The present invention relates to a focusing optical system for single-lens reflex cameras and, more particularly, to a focusing optical system to be used for detecting that the lens system of camera is focused at the time of automatic focusing or photoelectric focusing of single-lens reflex cameras.
(b) Description of the Prior Art:
Known focusing optical system for single-lens reflex cameras are arranged, for example, as shown in FIG. 1. That is, the light passed through a photographing lens 1 is reflected by a mirror 2, passes through a focusing glass 3 and enters a penta-prism 4. Out of the light entered the penta-prism as above, a part of said light is taken out through a surface 4a of the penta-prism 4 and is imaged on a detector 6 by means of a lens 5 for the purpose of detecting that the photographing lens 1 is focused. FIG. 2 shows another example of known focusing optical system. This example is arranged that, out of the light passed through the focusing glass 3, a part of said light is branched by a semi-transparent mirror 7 before said light enters the penta-prism 4 and is led to the detector 6 for the purpose of detection. FIG. 3 shows still another example of known focusing optical system. In this example, a semi-transparent mirror 3a is formed in the focusing glass 3 and the light branched by the semi-transparent mirror 3a is detected in the same way as the example shown in FIG. 2. In the above-mentioned known examples, the lens for imaging on the detector is arranged outside the penta-prism of single-lens reflex camera and, therefore, these examples have the disadvantage that the upper portion of camera becomes large in size. Moreover, in these examples, a part of light is branched halfway in the optical path of finder. Especially in the examples in which the semi-transparent mirror is arranged in the optical path up to the penta-prism as shown in FIGS. 2 and 3, the field of the finder becomes partially dark and it is inconvenient for observation through the finder. Furthermore, in these examples, the distance from the image A on the focusing glass 3 to the detector 6 is short and, therefore, magnification by the lens 5 arranged in front of the detector 6 becomes necessarily small. Consequently, longitudinal magnification becomes very small and the detecting accuracy for focusing becomes low. There is still another example of known focusing optical system in which two detectors are used for the purpose of detection. In this example, however, it is necessary to arrange the detectors outside the penta-prism like the afore-mentioned examples and, moreover, the lenses for imaging on the detectors should be arranged in front of the detectors. Therefore, in the example in which two detectors are used, a still larger space is required for arranging those lenses and detectors and, consequently, the upper portion of the camera becomes very large.